1. Field of the Invention
The present invention relates to a voltage-controlled variable-frequency pulse oscillator, and more particularly, to a pulse oscillator starts oscillation on its own when oscillation is interrupted due to turn on of the power supply or the like condition.
2. Description of the Prior Art
In recent times, voltage-controlled variable-frequency pulse oscillator of pulse feedback type is being utilized widely for various purposes. In a voltage-controlled variable-frequency pulse oscillator of this type, voltage of triangular wave form is generated by charging an integrating capacitor with a reference constant-current source and a voltage controllable current source, and then discharging the charged electricity through operation of a switching circuit which is connected to the capacitor. There are known oscillators of the kind in which the voltage thus generated is compared with a reference voltage by a comparator, and pulse signal with prescribed frequency is outputted by means of a monostable multivibrator.
The monostable multivibrator mentioned above is composed, for example, of an R-S flip-flop circuit obtained by connecting two OR-NOR gates in crisscross fashion. The OR output on the set side and the input on the reset side of the R-S flip-flop are connected via a delay circuit, and a voltage pulse is generated over the delay time of the delay circuit. The output terminals of the R-S flip-flop are connected to the switching circuit which is kept turned on for the duration of the pulse width of the voltage pulse. Discharge of the capacitor is carried out by the difference between the electric current of the reference constant-current source for discharge and the electric current of the current source for charging. Since the charging time of the capacitor varies according to the voltage control of the current source, the oscillator is constructed to have varying pulse frequency for the output of the R-S flip-flop in response to the controlling voltage of the voltage controllable current source.
However, when a rush current or an excessive noise current is generated in a voltage-controlled variable-frequency pulse oscillator due to turning the power supply or switch on, or like action, an unusually high voltage far exceeding the reference voltage, that is, the threshold voltage, for the comparator is charged to the capacitor, and is sent to the comparator. Accordingly, the inversion operation of the output of the comparator is suspended, allowing the capacitor to discharge for the period corresponding to the duration of just one pulse. Namely, a prior art pulse oscillartor of the above description gives rise to a problem that the repetition of discharge is discontinued at the time of throwing in the power supply or a switch, resulting in terminating the oscilation.
In order to resolve the above problem, a new oscillator has been proposed by the present applicant. This oscillator includes an R-S flip-flop in which a pair of OR-NOR gates are connected in the crisscross fashion where the output Q of the gate on the set input side is fed back, via delay circuit with prescribed delay time, to the reset input of the gate on the reset side.
To be more specific, this oscillator is disclosed in Japanese Patent-Publication No. 57-109416 which has already been filed by the applicant.
With an R-S flip-flop of the above construction, when both of the set input and the reset input are [1], the output Q becomes [1] while the output Q becomes [0]. In this case, even when the set input of the R-S flip-flop remains continuously at [1] due to suspension of oscillation, the output of the monostatic multivibrator stays at [1] also corresponding to the continuous valve [1] of the set input. Accordingly, discharge will be continued for some time until the voltage reaches the threshold value for the comparator when the output of the comparator is inverted, resuming the oscillation.
However, in a circuit construction like in the above, when the pulse width of the output signal from the comparator is larger than the ordinary width, ordinary oscillation is not possible because of an increase in the discharge time of the switching circuit.
That is, the width of the pulse from the comparator has to be smaller than the discharge time of the switching circuit in order to realize an ordinary oscillation. It is noted here, for making the oscillation frequency fs large, that it is necessary to choose the discharge time of the switching circuit to be smaller than the value determined by the general condition for sustaining oscillation. As explained in the foregoing, however, the pulse width of the comparator has to be smaller than the discharge time of the switching circuit so that there is a problem that the upper limit of the oscillation frequency need be restricted.
It should be mentioned further that there has been proposed the following as an alternative method for resolving the problem of discontinuation of oscillation. Namely, a method by which the capacitor is forced to discharge when an unusually high voltage exceeding the threshold of the comparator is applied to the capacitor at a time such as turning the power supply or a switch on. A pulse oscillator is formed by attaching a forced discharge circuit comprising a second comparator and a switching circuit for forcing the capacitor to discharge. The pulse oscillator discribed in the foregoing has been disclosed in Japanese Patent Disclosure No. 57-157624.
However, a pulse oscillator like the one described in the above requires an additional forced discharge circuit comprising a comparator and a switching circuit such that it was to lead to new problem like a complicated circuit construction, large size of the oscillator itself, increased cost for manufacturing the oscillator, and so forth.